It is believed that both the supply pin and the supply sleeve of every VHS videocassette now on the market are stainless steel. The Betamax videocassette operates at slower fast-forward and rewind speeds and lower tape tension, thus making it possible for a few companies to market Betamax videocassettes in which either or both of those supply tape guides is or are plastic, but it is believed that every such company recognizes the inferiority of plastic supply tape guides by offering at higher cost a Betamax videocassette in which both of the supply tape guides are stainless steel.
In order for a videocassette to meet recognized specifications, a stainless steel supply pin must have very smooth surfaces. A typical specification for the supply pin is based on peak-to-peak roughness measurements and permits no more than the following:
______________________________________ Peak size (micrometers) Max. No. of Peaks ______________________________________ 0.125 50 0.25 25 0.627 2 1.25 0 ______________________________________
A rougher surface would undesirably increase tape tension and might unduly scratch the recording layer of the tape. The attain a desirably smooth surface requires careful grinding. Furthermore, stainless steel inevitably is slightly magnetic, so that a stainless steel supply pin or sleeve may slightly degrade recorded signals, regardless of the degree of care used to avoid magnetism.
Because the backside of the tape rides on the supply sleeve, the surface of a stainless steel supply sleeve does not need to be as smooth as that of a stainless steel pin. An average peak-to-peak roughness of 0.3 micrometer or better meets typical specifications.
The use of stainless steel to make the supply sleeve involves a special problem in that a metal supply sleeve has a seam which should be positioned out of contact with the tape. To do this, each end of the supply sleeve is formed with a notch that fits over a v-lug in the base to keep the sleeve from rotating, hence keeping its seam out of contact with the tape. In a typical automated assembly line, mechanism at one station rotates the sleeve until the notch fits onto the lug, and mechanism at a second station checks that fit.
U.S. Pat. No. 4,365,769 (Shoji) says that videocassette tape guides (including the supply pin and supply sleeve) has been made of stainless steel coated with a hard chrome plating or a nickel plating. The patent concerns a nickel-tin alloy plating which is said to reduce the "static frictional coefficient" as compared to the prior platings.
German Offenlegungsschrift DE. No. 3218097 filed May 13, 1982 (assigned to TDK Electronic Co., as is the earlier filed Shoji patent) concerns such tape guides which it indicates may be stainless steel that is polished and lapped to have a smooth surface. It says that the guides are often damaged by contacting each other before being incorporated into videocassettes, attributing this to the fact that steel pins have a Mohs' hardness of about 5. Its answer is to make the guides from glass or ceramic which have a Mohs' hardness of about 7 and are much less susceptible to surface damage.